Definition:Differentiable Mapping/Complex Function/Point

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Let $D\subset \C$ be an open set.

Let $f : D \to \C$ be a complex function.

Let $z_0 \in D$ be a point in $D$.

Then $f$ is complex-differentiable at $z_0$ if and only if the limit:

$\displaystyle \lim_{h \to 0} \frac {f \left({z_0+h}\right) - f \left({z_0}\right)} h$

exists as a finite number.

This limit, if it exists, it is called the derivative of $f$ at $z_0$.